Combined Cycle Power Plants (CCPP) generally require preheating of a fluid medium to deliver Heat Recovery Steam Generators (HRSG) to up to or above a fixed minimum temperature required at the HRSG. Preheating of the fluid medium reduces the irreversibilities involved in steam generation in the HRSG and improves the thermodynamic efficiency of the CCPP. Among various others advantages of the preheating fluid medium, such preheating also helps avoid corrosion caused by flue gas condensation on outer tubes surface in inside the HRSG. Such preheating of the fluid medium may be achieved with or without feedwater heating system.
For the CCPP equipped without the feedwater heating systems, the preheating is normally achieved by recirculation of fluid medium recirculation from an economizer extraction in a dedicated HRSG coil at a cold end of the HRSG. A philosophy of the mentioned state of the art concepts is to control a fixed temperature, generally, of the fluid medium entering a low pressure economizer in HRSG in the feedwater tank.
Various configurations could realize this fluid medium heating concept, such as the one described in U.S. Pat. No. 6,427,636B1 (US'636). The described plant configuration is based on various control elements, as shown in FIG. 3 of US'636 (reproduced herein as Prior Art FIG. 4), incorporated in the fluid medium heating arrangement, such as, a first control element incorporated between the feed line and inlet line (10); a second control element incorporated between on the bypass line (11); and a third control element arranged on the recirculation line (12). US'636 also include several other process variant with additional control elements. Such various control elements throttles the flow to adjust the recirculation mass flow and therefore adjust the heat input to the fluid medium to maintain required temperature.
However, using such various control units and such regularized recirculation may constantly affect the CCPP efficiency. Such various control unit and recirculation must be carefully optimized for maximum CCPP efficiency.